Text("Hello",
     style: TextStyle(color: Colors.black, fontWeight: FontWeight.w900))

RichText(
  text: TextSpan(
    text: 'Hello ',
    style: TextStyle(color: Colors.black, fontWeight: FontWeight.w900)),
    children: <TextSpan>[
      TextSpan(text: 'bold', style: TextStyle(fontWeight: FontWeight.bold)),
      TextSpan(text: ' world!'),
    ],
  ),
)

Container(
            margin: const EdgeInsets.only(left: 20.0, right: 20.0, top: 10.0, bottom: 10.0),
            child: RichText(
              textAlign: TextAlign.center,
              text: TextSpan(
                children: [
                  TextSpan(
                    text: 'By clicking sign up, you have read and agreed to our ',
                    style: TextStyle(color: Colors.black54),
                  ),
                  TextSpan(
                    text: 'Terms & Conditions',
                    style: TextStyle(color: Colors.blue),
                    recognizer: TapGestureRecognizer()
                      ..onTap = () {
                        print('Tapped on hyperlink');
                      },
                  ),
                  TextSpan(
                    text: '.',
                    style: TextStyle(color: Colors.black54),
                  ),
                ],
              ),
            ),
          ),